Within hours of the launch, Word became the most downloaded application for iPads in Apple's app store.

The Excel and Powerpoint apps were the third and fourth most popular free app downloads, respectively, in the store.

The popularity suggests that customers are interested in accessing Microsoft's signature Office products in their new, easier to use incarnation, compared with the web-based alternatives provided before. But it is not yet clear how many will pay for a subscription to access all the apps' features.

The firm has faced criticism for not offering the software until now.

Mr Nadella said that the announcement was part of a strategy to empower people "to be productive across all devices" with Microsoft software.

Microsoft's new chief presented his first launch in San Francisco

"We are taking great focus and great care to make sure Office on any device shines through," he said, indicating that his firm would release versions of the apps for other mobile devices in the future.

Analysis

By Richard TaylorNorth America Technology Correspondent

Satya Nadella's move to bring Office to the iPad is significant as much for its symbolism as for the detail.

He has calculated that bringing Microsoft's flagship productivity software to a rival operating system is a better long-term bet than holding it hostage to Windows-based devices.

It is a bold statement of leadership, signalling a willingness to carve out a fresh direction for Microsoft.

It puts Office at the heart of this new strategy, one in which mobile and cloud software and services are key, and an openness to other platforms is embraced.

One risk in this cross-platform strategy is that iPad developers have filled an Office-shaped hole with apps which - for many users - are perfectly respectable alternatives.

There will definitely be a core of Office users excited by the prospect of a familiar app with streamlined integration into the desktop version through the cloud. The question is how large this paying audience will be.

Furthermore, if the move to iPad proves too well-executed, it may be at the expense of sales of its own Surface tablet and other Windows-based mobile hardware - one of the reasons the Microsoft old guard resisted for so long.

The new CEO is making it clear that Microsoft is moving in a post-Windows environment - what he calls "a mobile first, cloud first" world - and the software giant needs to adapt accordingly.

Research firm Gartner predicts about 271 million tablets will be shipped this year - only slightly less than its forecast of 277 million PCs and laptops - and Apple's iPad is currently the bestselling model.

Mr Nadella's predecessor, Steve Ballmer, launched an iPhone version of Office last year and confirmed an iPad version was in the works.

But many industry watchers have speculated that Mr Ballmer deliberately delayed its release in order to debut a tablet touch-centric version on Microsoft's own Surface machines before bringing it to a competing platform.

App rivals

Office remains Microsoft's cash cow, accounting for $16.2bn (£9.7bn) - or just over 60% - of Microsoft's operating profit in its last financial year. But some believe that sum could have been larger.

"It was definitely a major mistake to wait - an example of the insular old-world thinking of Steve Ballmer and his management team that believed everything should be within a Windows ecosystem," said Chris Green, from the Davies Murphy Group consultancy.

"In today's multi-device environment, where Windows is no longer the all dominant platform it once was, that game plan doesn't work anymore. The fact Microsoft is now catching up is only going to be a good thing and will be to the benefit of the Office applications."

The iPhone version has attracted a relatively low review score from Apple's App Store users, many of whom complained about its cost - it required an Office 365 subscription sold for £80 a year - and missing features.

Meanwhile other apps - including Documents to Go, HopTo, Quickoffice, Google's business web apps and Apple's iWork suite - have prospered offering free or cheaper alternatives that can load and alter files originally created by Office.

Apple and Microsoft's chief executives tweeted each other after the event

However, the iPad edition is more powerful than the original iPhone version - for example more complicated edits can now be made to Powerpoint presentations and the programs make recommendations to help create visually appealing documents - and several experts believe there will be strong demand for the product.

Freemium model

Users wanting to only view and present documents can use Office for iPad without charge, but an Office 365 subscription is still needed to edit them.

Small businesses are, however, offered a discounted annual rate of £39.60 per user for up to 25 workers.

"Much as you can edit Excel spreadsheets and tweak Word and Powerpoint documents with other software, it can involve technical gymnastics and be a great pain on an iPad," Richard Edwards, an analyst at tech research firm Ovum, told the BBC.

"Often when you move from one program to another the DNA of a document gets twisted and distorted, with formatting errors and other problems.

"For the reduction in stress people and businesses will be more than willing to pay for the subscription cost."

A laser-like graphic shows where the user's finger is pointing when an iPad running Powerpoint is linked to an external monitor

However, one analyst said Mr Nadella still had more work to do to reassure shareholders.

Excel for iPad makes recommendations to help users navigate through the app's menus

"The company's announcement today around offering Office for iPad - and eventually other devices - will be warmly welcomed by investors, in our view, as it adds a long awaited gateway to enterprise users, finally capitalising on bring-your-own-device-to-work trends," said Daniel Ives from FBR Capital Markets.

"[But] reinvigorating the Windows franchise continues to be a key ingredient in Microsoft's recipe for success.

"With the much-hyped Windows 8 having experienced lacklustre adoption, we believe investors will be looking for hints or previews around Windows 9 and subsequent updates to Windows 8.1 at the upcoming Build Conference as this struggling high-margin business remains front and centre."

Microsoft's Build conference for developers runs from 2 to 4 April in San Francisco.